A significant number of energy conservation programs have been adopted in the past recent years with the objective of reducing the energy consumption of electrical domestic appliances, primarily the heating system. One promising approach followed by the industry is the use of highly sophisticated electronic controllers capable of regulating the operation of the heating system in accordance with a variety of parameters.
Traditionally, electrical heating systems have been controlled by wall-mounted thermostats employing a switch for connecting or disconnecting the resistive heating elements of the heating system to the power line in dependence upon the ambient temperature. Such thermostats use a common bimetallic strip that curls when the temperature changes to mechanically actuate the switch toward the open/closed condition in order to control the power dissipation of the resistive load.
Perhaps the most obvious drawback of this simple thermostat design is the lack of automatic adjustment of the temperature set point. To achieve an efficient energy utilization while maintaining a certain level of comfort, the temperature in the room should be kept low when it is unoccupied and raised only when the room is populated. To achieve this control function with manually adjustable thermostats, the user is required to vary the temperature set point every time the occupancy of room changes, which of course is unpractical.
The electronic thermostats developed by the industry in the past recent years overcome this drawback. These devices employ programmable micro-processors that automatically adjust the temperature set point in accordance with a daily temperature evolution profile specified by the user. Typically, the user enters in the system memory datum of the desired temperature correlated to the time of the day. Once programmed, the micro-processor controls a load driver such as a relay or a power solid-state switch to regulate the operation of the resistive heating elements in order to maintain the ambient temperature as close as possible to the dynamic set point.
The electrical power required for the operation of the thermostat control circuitry can be supplied by a battery or furnished from the power line in series with the controlled load, the resistive heating elements for instance. The latter approach presents the advantage of reduced maintenance since no battery needs to be replaced. However, the power supply design is complicated in light of the requirement to extract electric energy from the power line when the latter is in different states of conduction. In this regard, it should be appreciated that a thermostat is normally installed on a wall where only two conductors are usually available, one of the conductors leading to a fuse or breaker in the switchboard of the dwelling and the other leading to the resistive heating elements. When the conductors are connected to one another, the power line loop is closed and current flows through the resistive elements. In contrast, when the conductors are disconnected from one another the electrical path of the power line is opened and the heating elements cease to function. The state of conduction of the power line is controlled by the load driver in the thermostat, either relay or solid-state switch, that is connected in series with the power line. When the load driver acquires an open condition, i.e. no current flows through the heating elements, the voltage supplied from the grid is present across the load driver terminals and provides a convenient source of power to supply the electronic control circuitry of the thermostat. This voltage can be easily stepped down, rectified, filtered and otherwise conditioned in accordance with the specific requirements. However, when the load driver assumes a closed condition, it establishes a quasi nil impedance path that only manifests an insignificant voltage drop resulting from parasitic resistive losses. Clearly, this potential is insufficient to furnish the controller circuitry with the required voltage and current for its operation. In this case, the power supply should be able to extract electric energy from the electrical current flowing in the power line.
One possibility to accomplish this objective is to provide a current transformer in series with the controlled load. The alternating current passing through the primary winding impresses via inductive coupling a current in the secondary winding. In turn, this current can be rectified and stored as a charge on a filter capacitor.
An important design criterion of the electronic thermostat is the ability to handle loads within a broad rating range. The manufacturer can thus commercialize a single model suitable for a wide variety of applications. In order to account for the different loads that the thermostat may be used with, the transformer provided to furnish the electronic control circuitry with electric power is selected to develop a sufficient voltage and current at the secondary winding when the primary winding is in series with the minimal load within the rating range of the thermostat. However, when the thermostat controls a load of increased capacity, a higher current flows in primary winding which, in turn, induces a higher secondary current. As a consequence, the value of the time integral of the voltage across the secondary winding may increase beyond the level at which the magnetic core of the transformer saturates and the core begins to emit objectionable audible noise of vibratory nature.